There was a little rebellion and God got pissy because he's got an ego problem and he cast Lucifer out. And mommy did not a damn thing to help her supposed favorite child.
It was all an excuse though, God wanted someone to rule over Hell (home sweet home) and who better than the Lightbringer? Worked out for me, it's how I was forged, in the fires of Hell to punish the guilty for all eternity. Good times.